"We are cutting by half the non-cereal part of the daily ration, starting in May," he said.
He added, "This way, we are hoping to eke out supplies so that they will last through July and August, which are the region's hunger months, as well as the rainy season."
According to a WFP statement issued on Friday, the reduction will not affect special feeding programmes for malnourished children and nursing mothers. However, WFP said this would have a significant impact on the diet of more than one million vulnerable people, reducing their daily intake of calories from 2,100 to 1,890.
"We are very concerned about the negative effect this drastic ration-cut will have on the health and psychological wellbeing of thousands of people - who are already weakened and traumatised by war," Carlos Veloso, the WFP emergency coordinator for Darfur, said.
"We have done everything to avoid this, including borrowing supplies - we are simply left with no alternative," he said.
WFP's statement added that although cereal donations to the agency had been generous, there had been little response to repeated appeals for non-cereals such as pulses, vegetable oil, sugar, salt and blended foods.
Smerdon said WFP had received just US $275 million, or 58 percent, of the $468 million it required to feed an average of 2.3 million every month in Darfur in 2005.
He said during the rainy season, the situation was likely to deteriorate as an additional 500,000 people were expected to require food aid.
Internally displaced persons (IDPs) were the main beneficiaries of WFP food aid, he added, although host communities also received food assistance, especially in localities where IDPs had moved into towns and mixed with the local population.
"We are trying to expand our assistance to host communities, but - although we are still hoping to do so - we currently do not have the funds to help them," Smerdon said.
Veloso said: "The people of Darfur need urgent aid. They don't have other options. The conflict in the region has robbed them of their homes and livelihoods. We have to do everything we can to make sure the assistance we provide meets their basic needs."
Conflict in Darfur pits Sudanese government troops and militias - allegedly allied to the government - against rebels fighting to end what they have called marginalisation and discrimination of the region's inhabitants by the state. At least 2.4 million people continue to be affected by the war, 1.85 million of whom are internally displaced or have been forced to flee to neighbouring Chad.